Swiss Patent 432,818 describes a process and apparatus for the coating of paper webs which impregnates a continuous paper web by passing it through a resin bath. The excess of the impregnating resin is then pressed out of the web in the nip between a pair of squeezing rolls. The coating mass or composition is then applied by a nozzle tube onto one surface, usually an upper surface, of the freshly impregnated still-wet paper web and is smoothed with a doctor strip. The so impregnated and coated paper web can then be carried through a dryer on a transport belt, cut into sheets and laminated as the cover sheet on a stack in a hot press.
In a similar process described in German Patent 195 08 797 C1, the upper web impregnated with the resin is dried to a predetermined original moisture content before it is coated with the composition on one side. After such coating the paper web is dried to a final moisture content. The resulting decorative paper is pressed to a laminate by applying it to a board of a wood composition.
The coating composition or mass may include a resin component which, by comparison with the melamine resin commonly used for lamination, has a significantly higher viscosity and is applied in a relatively thick layer in which the particles of hard material like corundum, can be uniformly distributed. One of the advantages of this process and especially the homogeneous distribution of the hard-material particles is that a smooth surface can be formed which is not interrupted by projecting hard material particles embedded in the composition and from which the hard material particles do not ablate.
This latter patent also notes that hard material particles which may emerge from the surface can cause increased wear of the tools used in processing the laminated products and, in particular, the press parts when hot pressing is used. Nevertheless press wear remains a significant problem when the coating mass or composition contacts the press parts and the earlier process is practiced.